Thanks to new directory software, Windows Vista could put a greater load on Internet servers. But experts disagree over whether we're headed for a prime-time traffic jam or insignificant slowdown.

Microsoft's launch of Windows Vista could slow down or stall traffic on the Net, said Paul Mockapetris, who is widely credited with inventing the Internet's Domain Name System (DNS). Mockapetris believes Vista's introduction will cause a surge in DNS traffic because the operating system supports two versions of the Internet Protocol, a technology standard used to send information over computer networks.

"If you adopt Vista, your DNS traffic is going to double," Mockapetris said in an interview. With many DNS servers already running close to capacity, this can have serious consequences, he said. "You're going to see brownouts. All of a sudden, it is going to be mud season on the Internet, where things will just be kind of slow and gooey."

Vista may cause an increase in DNS traffic, but not to the extent predicted by Mockapetris, Microsoft countered in a statement provided to CNET News.com last week. Other experts support Microsoft and suggest Mockapetris' predictions are related to his role at Nominum, the Redwood City, Calif., vendor of DNS products where he is chief scientist.

Others agree that Vista could cause a spike in DNS traffic. But they're not expecting dire consequences. "Vista, due to its support for IPv6, will cause somewhat higher load on name servers as it checks to see which protocol to use," said Dan Kaminsky, an independent researcher. "But this is not the stuff that blackouts are made of."

Vista is the first Windows version to support the new IP version 6, which is designed to provide a broader range of IP addresses. As current IPv4 addresses are becoming scarce, IPv6 will provide easier connectivity across the Internet and remove the need for IPv4-addressing schemes such as network address translation, which can require additional management burdens and cause application incompatibilities.

But IPv6 is far from being universally used. So, Vista will also support the current IPv4. The side effect, according to Mockapetris, is that a Vista PC will make two DNS requests, one for each IP version, instead of just one.

"It is going to try a DNS lookup for the IPv6 address and then a DNS lookup for the IPv4 address," Mockapetris said. "It just uses more DNS, and until we increase the supply, things are going to go slower."

DNS is crucial to the Internet. It functions as a phone book, mapping text-based addresses such as www.cnet.com to the actual numeric IP address. DNS servers are typically run by Internet service providers, hosting companies and larger businesses that have Net connectivity.

Nominum commissioned a survey of the DNS servers run by large broadband ISPs. "It looks like they are right at the knee and curve and if Vista was all of a sudden deployed everywhere, we'd be having rolling blackouts," Mockapetris said. "For my home network, it is not going to matter, but for these people with millions of users, it is going to matter."

Vista's use of IPv6 will not disrupt the Internet at large, said David Ulevitch, chief executive at OpenDNS, a provider of free DNS services. "DNS can be improved, but predicting its collapse is just spreading FUD (fear, uncertainty and doubt)."

While there are name servers that are running close to capacity, Kaminsky, in a preliminary scan, said he found most networks have quite a bit of spare capacity. "Vista cuts into some of the slack space already in place for shock load," he said.

The DNS system is relatively complex. Vista, in fact, won't query twice every time it sends out a DNS request, Microsoft said. There will be some more traffic, but the Redmond, Wash.-based software maker does not expect Vista machines will impact the overall functionality of the Internet, the company said.

"We feel we have designed our implementation of DNS to be very efficient by querying twice only when absolutely necessary," Microsoft said. "In our beta deployments with enterprises, we have not found the DNS queries are resource intensive and do not believe DNS queries from Windows Vista machines will cause a large surge in queries overall."

For example, Microsoft designed Vista so PCs will query in the address of the type assigned to the system, the company said. Computers that don't have an IPv6 address will not do IPv6 queries, the company said. Also, when a machine does do an IPv6 query, it will do so only to a DNS server that responded to its initial IPv4 query, the company said. "Name errors are not repeated, so the Net traffic will less than double," it said.

At least one of the technologies related to IPv6 in Vista was changed recently, after the release of Beta 2 in May, Microsoft said. Vista will now acquire a so-called Teredo address only when an application requires its use. Teredo is a tunneling protocol to use IPv6 with networking gear such as certain routers.

Nevertheless, an eye should be kept on bottlenecks such as DNS forwarders, name servers at ISPs that handle queries from thousands of broadband customers, said Cricket Liu, a DNS expert and vice president of architecture at DNS appliance maker Infoblox.

Still, Liu calls Mockapetris' prediction of brownouts "a little alarmist." A major factor will be the speed at which Vista is adopted and while Microsoft might like to see an overnight worldwide upgrade, most pundits expect Vista's adoption to be gradual, even slow.

Representatives for Comcast, Verizon Communications and EarthLink, all companies with a high-speed Internet access business, were not available to comment on this story.

The worst-case scenario, Kaminsky said, is a couple of spot failures occuring during peak hours at select organizations. "The one exception might be a few enterprise customers running really close to the redline on their name servers. I could see checking for spare capacity on the name server if 50,000 systems got moved over to Vista overnight." Such a big migration, however, is rather unlikely, he added.

Already millions or people are running trial versions of Vista and Microsoft is working with ISPs as well as part of its beta program, the company said. Vista is slated to be broadly available in January. Microsoft only last week made a near-final trial version of the operating system available to testers and said Tuesday that the program will eventually be expanded to roughly 5 million testers.

Mockapetris, responding to Microsoft, said the company appears to have "a good, if static, strategy." Yet, he questioned it. "Some reports have claimed to observe different behavior; it might be that Microsoft changed the strategy as they fine-tuned things."